Origin of the monoclinic-to-monoclinic phase transition and evidence for the centrosymmetric crystal structure of BiMnO₃

Structural properties of polycrystalline single-phased BiMnO₃ samples prepared at 6 GPa and 1383 K have been studied by selected area electron diffraction (SAED), convergent beam electron diffraction (CBED), and the Rietveld method using neutron diffraction data measured at 300 and 550 K. The SAED and CBED data showed that BiMnO₃ crystallizes in the centrosymmetric space group C2/c at 300 K. The crystallographic data are a = 9.5415(2) Å, b = 5.61263(8) Å, c = 9.8632(2) Å, β = 110.6584(12)° at 300 K and a = 9.5866(3) Å, b = 5.59903(15) Å, c = 9.7427(3) Å, β = 108.601(2)° at 550 K, Z = 8, space group C2/c. The analysis of Mn-O bond lengths suggested that the orbital order present in BiMnO₃ at 300 K melts above T₀₀ = 474 K. The phase transition at 474 K is of the first order and accompanied by a jump of magnetization and small changes of the effective magnetic moment and Weiss temperature, μ_eff = 4.69μ_B and θ = 138.0 K at 300-450 K and μ_eff = 4.79μ_B and 0 = 132.6 K at 480-600 K.